Dual path vehicles require two hydrostatic transmissions per vehicle. One transmission drives the left propel wheels or track, and the other transmission drives the right propel wheels or track. Each transmission has a separate control lever to stroke the transmission for the operator command station. Thus, if the operator pushes both control levers forward an equal amount, the operator expects both transmissions to provide the same output so the vehicle will go straight ahead. If one transmission provides a greater output than the other, the vehicle will began to steer to one side or the other. The symmetry from the left to the right transmission is the most important when the operator is leaving neutral and when the operator reaches full stroke. Near neutral is important because if the operator is trying to inch forward very precisely, both transmissions should (left propel and right propel) start moving at the same time. If they do not move at the same time the vehicle will index to one side making it very difficult to do precise inching. Likewise with the handle at maximum angle, both transmissions need to have the same output or the vehicle will not go straight, but tend to steer opposite to the transmission with the greatest output.
Symmetry is also important on single mode transmissions to a lesser degree. The user expects equal output from the transmission regardless of whether the handle is moved in the forward or reverse direction.
To meet desired requirements of a larger neutral dead band and smooth control near the neutral position, it is desirable to incorporate a cam to tailor the movement of the valve spool relative to input handle. This tailored movement will optimize the vehicle controllability. It is also desirable to spring center the input handle to isolate any clearance or backlash in the vehicle control linkage from affecting the performance of the transmission.
However, spring centering the handle can create a problem with symmetry from forward to reverse. If the transmission control linkage or hydraulic valve spool are adjusted to ensure that the spool is centered in the neutral porting position, it does not assure that the handle is centered in the cam dead band. Likewise, if the handle is centered, this does not assure that the spool is centered.
When such vehicles are going downhill, the hydraulic motor in the transmission starts pumping oil to the pump. This backlash action causes the motor to override the pump and the vehicle speeds up. This can be counteracted by the valve spool in the hydraulic control, but the spool is slow to react because of accumulated tolerances in the control linkages and in the linkage on the equipment.
It is therefore a principal object of this invention to symmetrically center the control handle of the hydrostatic transmission control, and to eliminate all accumulated tolerances in the linkage system.
A further object of this invention is to provide a method of assembly of the hydraulic transmission controls which will precisely center the control lever in the center of the dead band.
These and other objects will be apparent to those skilled in the art.